High-pressure fuel pumps for common rail fuel injection systems typically comprise one or more hydraulic pump heads in which fuel is pressurised in a pumping chamber of the pump head by the reciprocating movement of a plunger.
Typically, low-pressure fuel is delivered to the pump head by a low-pressure lift pump in the fuel tank and/or by a transfer pump built into the high-pressure fuel pump. The low-pressure fuel is drawn into the pumping chamber through an associated inlet valve on a filling or return stroke of the plunger, during which the volume of the pumping chamber increases. On a pumping or forward stroke of the plunger, the inlet valve closes and the volume of the pumping chamber decreases, resulting in an increase in the fuel pressure within the pumping chamber. At a pre-determined pressure, an outlet valve associated with the pumping chamber opens to allow the high-pressure fuel out of from the pumping chamber to the common rail for delivery to the fuel injectors.
The fuel pressure in the common rail, which determines the fuel injection pressure, may be varied from a moderate pressure of a few hundred bar at low engine loads and speeds to a very high pressure of up to 3000 bar or more at high engine loads and speeds.
To regulate the fuel pressure in the common rail, an additional control valve, known as an inlet metering valve, may be provided upstream of the inlet valve of the pumping chamber. The inlet metering valve is used to control the amount of fuel that enters the pumping chambers of the fuel pump, and consequently the quantity of fuel that is compressed and delivered to the common rail at high pressure with each plunger stroke.
A conventional inlet metering valve is effectively a controllable orifice, which acts to throttle the flow of fuel to the inlet valve of the high-pressure pump. In this way, only the amount of fuel required by the engine is delivered to the rail, thereby saving both fuel and energy compared to the situation where fuel is fed by the lift or transfer pump at constant full delivery. The inlet metering valve is under the control of the engine control unit, which determines the desired rail pressure and the actual rail pressure and adjusts the inlet metering valve accordingly.
There are several disadvantages in the use of conventional inlet metering valves. In particular, inlet metering valves can be expensive and add to the overall cost of the common rail injection system, which is undesirable. Secondly, inlet metering valves are relatively large and space-consuming components. Thirdly, inlet metering valves can be vulnerable to wear and to damage due to low-quality fuels. Furthermore, in some arrangements, the use of a conventional inlet metering valve means that the metering/rail pressure control mechanism is relatively far from the pumping chamber of the high-pressure fuel pump, which leads to undesirable delays in rail pressure control.
In an alternative arrangement, the inlet valve for the pumping chamber is provided with an actuator arrangement which allows the inlet valve to be closed in response to a signal from the engine's electronic control unit. In this way, the quantity of fuel that enters the pumping chamber during the filling stroke of the plunger can be regulated without the need for an additional inlet metering valve. Such arrangements are described in DE 10 2008 018018 and EP 1921307. EP 1921307 also describes the use of the inlet valve as a spill valve to return high-pressure fuel from the pumping chamber to the fuel rail during the pumping stroke of the plunger.
Typically, electronically-controllable or switchable inlet valves are actuated by a solenoid actuator arrangement operable to control the movement of a poppet-type inlet valve member that is received within a bore in the pump head. An armature is attached to a valve stem of the valve member, and a head portion of the valve member is engageable with an associated seat formed at the end of the bore. When the solenoid is energised, the armature is drawn towards a core of the solenoid against the force of a biasing spring, which biases the valve stem into a normally-open position.
In practice, the performance of such solenoid-actuated inlet valves can be compromised by several factors. For example, the inlet valve is in its fully-open position, it is desirable that the cross-sectional area available for fuel to flow between the valve head and the valve seat is as large as possible, to maximise the flow of fuel into the pumping chamber at high engine loads. For this reason, the stroke of the valve member between its fully-open position and its fully-closed position must be relatively long. This, in turn, means that the air gap between the armature and the core is relatively large when the valve is fully open. Since the force applied to the armature of a solenoid actuator decreases significantly as the air gap increases, a relatively large and expensive solenoid must be used to achieve the force necessary to close the valve.
Furthermore, as a result of wear of the valve member and/or the valve seat, the stroke of the valve member between its fully-open and fully-closed positions can vary over time, resulting in variations in the air gap between the armature and the core over the service life of the inlet valve. Similarly, when a lift stop arrangement is provided to limit movement of the valve member beyond its fully-open position, wear of the lift stop arrangement can result in an increased air gap when the valve member is fully open. Thus the force required to close the valve can vary during use of the valve, which can reduce the performance and controllability of the valve arrangement.
Also, in such an arrangement, the armature is typically in the form of a collar that is press-fitted or otherwise attached to the valve stem. Any variation in concentricity between the armature and the valve stem, and between the armature and the core, can result in undesirable side-loads that can cause excessive wear of the valve member and the valve seat during the service life of the inlet valve. Because the inlet valve is subject to very high fuel pressures, such wear can seriously impair the performance and reliability of the valve. The inlet valve must therefore be manufactured with very tightly-controlled tolerances in the dimensions and concentricity of the parts, which increase manufacturing complexity and cost.
Against this background, it would be desirable to provide an electronically-controllable inlet valve assembly for the pump head of a high-pressure fuel pump which substantially overcomes or mitigates at least some of the above-mentioned problems.